1. Field of the Invention
The present invention is broadly concerned with improved solids removal assemblies used in various types of sedimentation tank systems in order to more efficiently remove solids from incoming waste water streams. More particularly, the invention is concerned with such solids removal assemblies, as well as methods of operation and complete solids removal systems incorporating the solids removal assemblies. The preferred solids removal assemblies include an elongated hood having converging sidewalls together with an elongated fill structure located within the housing; a plurality of upright outlet pipes are operatively coupled to the hood for withdrawal of liquid-solid suspensions drawn into the hood, permitting downstream processing of the suspension.
2. Description of the Prior Art
A sedimentation tank forming a part of sewage or other waste water treatment facility is typically a tank designed to receive incoming waste water-solids suspensions, wherein the suspension is allowed to remain under quiescent conditions permitting gravitation of the solids to the bottom of the tank. Solids-free liquid is then withdrawn from the surface of the tank, while solids are withdrawn from the bottom. Such sedimentation may occur with a flow-through sedimentation tank where a waste water-solids suspension is continuously introduced and solids and liquids are continuously withdrawn. Alternately, the sedimentation tank may be of a batch variety where the tank is filled with the suspension and solids and liquid are withdrawn in batches.
In sedimentation tanks of both types, the clear liquid is typically skimmed or discharged from the surface of the tank by displacement. This generally can be readily accomplished. However, removal of the solids presents a more difficult problem. Some solids, such as sand-type materials, will settle rapidly and normally require some form of physical scraping to remove them from the bottom of the tank. With these rapidly settling solids, disturbance of the solids by the removal device or method is usually not a problem, as the solids quickly resettle after disturbance. Other solids, such as chemical or biological flocculants, are very lightweight and settle slowly. They are accordingly very susceptible to resuspension by any turbulence caused by a sludge or solids removal mechanism at the base of the sedimentation tank.
In flow-through sedimentation tanks, removal of solids must occur on a near-continuous basis, or else the tank will ultimately fill with solids. However, it is also necessary to provide conditions which are as quiescent as possible to prevent resuspension of solids back into the clarified liquid. Again, this is particularly troublesome in the case of lightweight flocs.
The prior art has developed a number of approaches for solids removal from sedimentation tanks. As indicated, physical removal techniques have included drag arms attached to a drive mechanism and operable to physically pull the solids from the bottom of the sedimentation tank to a central collection point, where they are either removed by gravity or by pumping. Another method used in circular sedimentation tanks is to employ a rotating device that pivots around the center of the tank. Such a device includes plow-like scrapers that physically scrape settled material to a central hopper where it is removed by gravity or pumping. Such devices have the advantage of positive removal of solids, but suffer from the disadvantage of physically moving a scraper or the like through already settled materials. When the material is light and fluffy, this movement resuspends the solids which will slip over the top of the scrapers and also create undesirable currents within the sedimentation zone.
In order to ameliorate the effects of physical scraping and plowing, some systems include tubes attached to the scraping devices in order to suction the material. This does offer some advantages of the lightweight materials; however, a structure which must move through the settled solids is still required.
Another type of solids removal device includes structure at the bottom of the sedimentation tank so that the solids will settle down slopes and collect in a hopper. Typically, a mechanical or airlift pump is placed near the bottom of the hopper and is operated continuously or intermittently to pump solids out of the hopper. This system has the disadvantage that in order to create slopes for solids to settle, the surfaces need to be at least 60° relative to the horizontal, and the solids need to be smooth to facilitate collection thereof. There becomes a practical limit on how deep this hopper can be because the hopper extends upwardly into the sedimentation tank at an angle of 60° or more.
Another disadvantage of this type of system is that the settled solids tend to “bridge” within the hopper zone, with the pump then removing only clear liquid while the solids compact and build up above the “bridge.” In order to keep this from happening, the pump is often run continuously to prevent bridging. However, when this is done, too much liquid is removed with the solids. When the solids are removed from the excess liquid during downstream processing, the removed liquid must be returned to the sedimentation tank, increasing the hydraulic load on the system and decreasing the capacity thereof.